I'll be missing it here due to Massachusetts' recent lamentable imitation of Seattle weather, but today marks the last Transit of Venus until 2117. What folks without rain will see is the black dot of Venus travelling across the sun as the planet passes between Earth and our star.
I learned more than I ever knew possible about the Transit of Venus and how astronomers used it to measure the size of the Solar System when I read Thomas Pynchon's excellent historical novel Mason & Dixon. Turns out the two Englishman went to measure the Transit of Venus in South Africa in 1761 before their Pennsylvania/Maryland border measuring days. Read a relevant excerpt here, and more on today's transit here.